Is handwritten asm more dangerous than compiled C code?

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fasmnewbie

Joined: 01 Mar 2011
Posts: 555

fasmnewbie 26 Sep 2018, 18:38

And btw, if one think that C++ is free from "mysterious" behavior, then listen to this guy...

https://www.youtube.com/watch?v=3MB2iiCkGxg

26 Sep 2018, 18:38

Furs

Joined: 04 Mar 2016
Posts: 2561

Furs 27 Sep 2018, 17:39

fasmnewbie wrote:

It's not dangerous per se, but rather it is easier to make mistakes (or intentional 'mistakes') in ASM if one is not careful enough. A pseudo example;
Code:
mov rdi,0x007ce0fffffffe    ;a pointer to some string variable
add edi,2                   ;oops!    
Looks syntactically / arithmetically legit, but its a big no. You can give this bug any name you think fit because it fits into any types of bugs you can think of. This is no undefined behavior. It is simply being rude

It's similar in C/C++ for different reasons though. This is a problem on 64-bit only, because int is not the same size as pointers anymore.

And every noob programmer always uses int everywhere or was taught like that from retards.

64-bit only issues in C/C++ that a LOT of people make:

Subtract two pointers and store in int? Security Vulnerability.
Many people also use something like ~0u to store a mask of "all 1s" in a type. But ~ is unary operator, and 0u is an unsigned int. So when it gets zero-extended to 64-bits (because it's unsigned int) you'll get only the lower half of 1s.
Using int for array indices of arbitrary input and size.

27 Sep 2018, 17:39

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20445
Location: In your JS exploiting you and your system

revolution 27 Sep 2018, 18:01

I find in my assembly code that signed integers are almost never the correct type. Unsigned, using ja, jb, etc. is almost always the test I need.

If C made the amount of typing needed to specify unsigned, the same as signed (i.e. three characters i n t) then perhaps unsigned would be used more.

People are basically lazy so if it needs more typing then it gets ignored.

27 Sep 2018, 18:01

Furs

Joined: 04 Mar 2016
Posts: 2561

Furs 27 Sep 2018, 20:39

Oh that's another topic where I fully agree with you. It's about signed vs unsigned.

The "signed everywhere" mantra is one of the most annoying I've ever laid my eyes on. I'm personally sick of polluting the beauty of code just to cater to a bunch of retards who might assume that signed values are "available", and unfortunately you got some top figureheads in C++ committee and Google who preach this crap.

It's nice to see a few other people are of same mindset, so I'm not alone, cause it sometimes feels that way. This blog post is also on that topic: https://blackhole12.com/blog/signed-integers-considered-stupid-like/ Smile

27 Sep 2018, 20:39

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 27 Sep 2018, 22:26

One more funny thing is when students come to C after two other languages (x86 assembly included) and find out that char is an integer type (not a problem) and is usually signed by default (not a problem as well). The problem is negative values in the debugger, very convenient in the world where most encoding tables are either unsigned or hexadecimal. Anyway, they’re aware that 'a' is of type int by that time and that arrays are not first-class citizens, so they’re not surprised.

27 Sep 2018, 22:26

Melissa

Joined: 12 Apr 2012
Posts: 125

Melissa 28 Sep 2018, 06:14

I see long debates about integer overflow when talking about C, as that is also undefined behavior ;p

28 Sep 2018, 06:14

Furs

Joined: 04 Mar 2016
Posts: 2561

Furs 28 Sep 2018, 13:51

Melissa wrote:

I see long debates about integer overflow when talking about C, as that is also undefined behavior ;p

Only on signed types.

Unsigned overflow is perfectly defined (just like asm).

28 Sep 2018, 13:51

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